Good points made by others. But another thing is you don't have to? Nix(OS)/Guix installs packages and dependencies separately instead of replacing packages like other OS/package managers do.

This means you have multiple versions of a package installed in your system and that you can use them simultaneously for different applications if I understand it right. You can move back and forth between versions if one breaks without effecting the rest of the system. Also if I remember, nixos-update rebuild switch (or something very similar to this command) which is usually fired after installing new packages or when a major change is made to the nix configuratino file creates a new boot entry and hence a snapshot for you to go back to if something in NixOS breaks. So there is no necessity for an LTS version to be present for stability's sake.

Garbage collection is also left to the end user to deal with. There is a garbage collector command in Nix package manager which will clean up when you push the command so that new packages are not flooding your storage space.

NixOS's stable channel can be very loosely compared to Manjaro in the sense that just like Manjaro, NixOS's Stable repo does opinionated changes/interferences/fixes. And NixOS Unstable is like Arch linux with latest and greatest upstream stable versions of the packages.